1.1 Field of the Invention
The present invention is generally directed to the analysis of underground earth formations, and, more particularly, to the determination of formation resistivity properties and/or profiles.
1.2 Description of Related Art
Knowledge of the resistivity properties of underground earth formations is fundamental in the evaluation and characterization of potential and existing hydrocarbon-bearing reservoirs. The terms “logging” and “log” are respectively used herein to designate any sequential measurements and the recording of such measurements as a function of depth and/or time of variations in a given characteristic of the formations around a borehole.
Formation resistivity anisotropy is a characteristic of reservoirs that can complicate formation evaluation. Many reservoir rocks exhibit resistivity anisotropy, especially when saturated with oil. There are several mechanisms, which can produce this anisotropy, among which are very thin sand-shale laminations, depositional changes in clean sandstone, and wind-distributed sands (aeolian formations). See Rubin, D. M., Cross bedding, bedforms, and paleocurrents, SOCIETY OF ECONOMIC PALEONTOLOGISTS AND MINERALOGISTS, CONCEPTS IN SEDIMENTOLOGY AND PALEONTOLOGY, 1; Klein et al., The petrophysics of electrically anisotropic reservoirs, Transactions of the SPWLA THIRTY-SIXTH ANNUAL LOGGING SYMPOSIUM, Paris, France, Jun. 26–29, 1995, paper HH. Resistivity measurements from within a borehole drilled through these formations have been made by a number of techniques employing the use of well-known instruments, including induction and laterolog (electrode) type devices.
In induction logging, a transmitter coil mounted on a well tool is lowered into a borehole and energized by an alternating current. Indications are then obtained of the influence of surrounding formations on the electromagnetic field established by the coil. Usually such indications are obtained by observing the voltage induced in a receiver coil also mounted on the well tool in coaxial relationship with the transmitter coil and axially spaced apart therefrom. Conventional induction well tools include at least one transmitter and a receiver coil array. Such an induction tool is illustrated and described in U.S. Pat. No. 5,157,605, assigned to the present assignee.
The laterolog technique comprises making a measurement with a well tool having a current emitting electrode and voltage electrodes one either side thereof which are operated to force current into the formation. The current is focused into the formations and returns locally, i.e., to the tool body. The formation resistivity is determined by measuring the current flow. Conventional laterolog tools are illustrated and described in U.S. Pats. Nos. 5,754,050, 5,852,363, 6,023,168, and Documents EP-478409, EP-544583, all assigned to the present assignee. Document EP-544583 describes a laterolog tool including both a “deep” investigation mode and a “shallow” (LLS) investigation mode.
The sensitivity of a resistivity logging tool to resistivity anisotropy depends on the physics of the measurement and the relative dip angle of the well bore of the formation. For example, 2 MHz induction-type tools have no sensitivity to resistivity anisotropy when the formation layers are perpendicular to the well bore, but will be quite sensitive at relative dip angles above 60 degrees. See Lüling et al., Processing and modeling 2 MHz resistivity tools in dipping, laminated, anisotropic formations, Transactions of the SPWLA THIRTY-FIFTH ANNUAL LOGGING SYMPOSIUM, Tulsa, Okla., Jun. 19–22, 1994, paper QQ.
Induction logging tools are insensitive to resistivity anisotropy at low relative dip. The current density set up in the formation is limited to the horizontal direction. See Moran et al., Effects of formation anisotropy on resistivity-logging measurements, GEOPHYSICS, V. 44, No. 7, July 1979, 1266–86; Anderson et al., The Response of Induction Tools to Dipping Anisotropic Formations, Transactions of the SPWLA THIRTY-SIXTH ANNUAL LOGGING SYMPOSIUM, Paris, France, Jun. 26–29, 1995, paper D. At higher relative dips the measured resistivity will combine both vertical and horizontal components, However, the effect does not depend on the coil array spacing, so the components are not resolvable. See Anderson et al., The Response of Induction Tools to Dipping Anisotropic Formations, Transactions of the SPWLA THIRTY-SIXTH ANNUAL LOGGING SYMPOSIUM, Paris, France, June 26–29.
Laterolog logging tools have some sensitivity to the vertical component of resistivity. See Moran et al., Effects of formation anisotropy on resistivity-logging measurements, GEOPHYSICS, V. 44, No. 7, July 1979, 1266–86; Chemali et al., The effect of shale anisotropy on focused resistivity devices, Transactions of the SPWLA TWENTY-EIGHTH ANNUAL LOGGING SYMPOSIUM, London, England, June 30–Jul. 2, 1987, paper H. However, with the dual-laterolog, the deep laterolog was not very sensitive to anisotropy. The shallow laterolog, which returns current to nearby electrodes, is more sensitive to anisotropy (See Moran et al., 1979), but invasion will reduce the effect of anisotropy.
It is desirable to obtain a reliable method and system for determining the resistivity properties and profiles of potential hydrocarbon-bearing zones in subsurface formations. Thus, there remains a need for improved methods and techniques to indicate formation resistivity anisotropy and evaluate the vertical component of resistivity.